The impact of tropical milkweed is a complicated topic. The plant has been banned from sale in Marin, Contra Costa, San Mateo, and Ventura counties after being given a "B" pest rating by the CDFA, partly due to its potential impact on monarch butterflies. The CDFA rating does not require sales to be prohibited, and the counties that have enacted restrictions do not require the removal of existing plants.

However, there is controversy about the impact of topical milkweed on monarch butterflies. The issue is complex, and recommendations vary based on the source. The controversy stems from the effects of a protozoan parasite, Ophryocystis elektroscirrha or OE, and its impact during winter. Since native milkweeds go dormant during the winter and tropical milkweed does not, there has been some concern that tropical milkweed encourages monarchs to breed locally, instead of migrating, and increasing the risk of OE infection.

A suggested mitigation strategy has been to cut down tropical milkweed during the winter as a preventative measure. However, it's unclear whether this is necessary, and additional scientific study is needed. The most recent UC blog post we know of, titled About That Tropical Milkweed..., has more information and includes comments from UC scientists. 

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